For many years, zinc dialkyl dithiophosphates (ZDDP) have been used as antiwear/antioxidant additives in lubricant oils. However, the use of ZDDP is accompanied by a number of substantial drawbacks, e.g., (1) ash formation, (2) glazing of the exhaust catalysts by zinc pyrophosphate, a result of noncombusted ZDDP in the blowby, and (3) antagonistic interactions between ZDDP and other additives such as the succinimide polyamine dispersants.
It has been known for some time that a hydrocarbon containing a sulfur atom in the backbone of the hydrocarbon chain and ending with a polar group will exhibit wear mitigating properties. See for example B. A. Baldwin, ASLE Trans, 1985, 28, PP 381-8. See also European Pat. No. 165670 and U.S. Pat. No. 4,217,233. The European patent describes a thio ester prepared by the reaction of a mercaptoethanol with coconut oil and the above U.S. patent provides an acetate prepared from the reaction from episulfide with mercaptoglycolate.
It is also known that cetain N,N-disubstituted amides are effective in lubricant base stocks. See U.S. Pat. No. 3,312,620.
Furthermore, as described in U.S. Pat. No. 2,673,839, condensation products of a thiocarbamic acid or salt thereof and a sulfide of a halogen substituted low molecular weight, saturated fatty acid halide and a carboxylic ester will inhibit oxidation in motor oil and are effective in dispersing sludge. As is well known, oxidation products in motor oil can be corrosive and cause excessive wear of metal engine parts. The presence of the halogen however is required in the sulfide in order to form the condensation product. There is no indication in that patent that an unsubstituted fatty acid sulfide ester alone would be effective for any purpose.